The present invention relates generally to electronic control systems for air conditioners, heat pumps and refrigeration equipment. More particularly, the invention relates to a control system using a multiple microprocessor, distributed architecture configuration for controlling refrigerant flow, fan speed and other components, based on environmental measurements.
The heating and cooling industry has been striving for some time to improve the efficiency of air-conditioning and heat pump systems. At the same time, there has been trend of adding new features and systems to increase occupant comfort and ease of use.
System efficiency is a multifaceted concept, with a number of different ways of measuring and evaluating efficiency. Currently in the United States the parameters by which efficiency may be measured and evaluated are established by the Air-Conditioning and Refrigeration Institute (ARI). To allow different makes and models of air-conditioning and heat pump equipment to be compared for efficiency, ASHRAE has promulgated the efficiency measurement standard set forth in ARI 210-81. By those standards, for example, efficiency is measured at the unit's full rated output, i.e., with the unit operating at maximum heating or cooling effort. While this method of rating efficiency does allow units of different manufacturers to be compared on a somewhat common ground, in day to day operation, most heating and cooling equipment cannot be expected to operate continuously at full rated output. In fact, we have found that operating heating and cooling equipment at all times at full rated output actually degrades system efficiency. At many times during the day, and in some regions of the country at nearly all times, optimal efficiency is achieved at well below the maximum rated output. In these regions, energy could actually be saved if the heating and cooling equipment were scaled back to operate below peak output.
To do so, however, is not simply a matter of adjusting the thermostat up or down a few degrees, or scaling back the amount of refrigerant pumped through the system. Heating and cooling systems which have been designed to operate at optimum efficiency during full rated output will not necessarily operate at optimum efficiency when scaled back to less than full rated output. Optimal efficiency is ordinarily fine-tuned into the system during the design and manufacturing process, based on a wide variety of different physical and thermodynamic constraints which are not readily alterable after the system is built. Thus it has heretofore been impractical and uneconomical to provide heating and cooling systems with the ability to optimize efficiency over the normally encountered range of operating limits. Because ARI standards base efficiency ratings on full rated output, heating and cooling systems are understandably designed to provide optimum efficiency at full rated output. In this way the systems will compare favorably with their competition. The result, of course, is that systems, which rarely if ever need to operate at full rated output, rarely or never achieve optimal efficiency.
Compounding the efficiency problem is the issue of comfort. To a considerable extent, relative humidity affects comfort. When conditions are dry one feels comfortable at higher temperatures than when conditions are humid. In humid weather it is often necessary for the building occupant to lower the thermostat below the normal setting just to be comfortable. One benefit of air-conditioning systems is that they remove moisture from the air being cooled.
Surprisingly, however, many highly efficient air-conditioning systems remove less moisture from the air than older, less efficient ones do. This is due to the fact that a highly efficient air-conditioning system nominally operates at higher evaporator coil temperature, which removes less moisture from the air passing over it. The net effect is that sometimes a highly efficient air-condition system ends up consuming more power than an inefficient one, because the user may set the thermostat lower in order to feel comfortable. Present-day air-conditioning systems have not adequately addressed this problem.
Another area where improvements are wanted is in the manner in which refrigerant is metered through the system. The refrigerant cools by evaporation in a heat exchanger commonly called an evaporator coil. The refrigerant is metered to the evaporator coil through an orifice sometimes called an expansion valve. Ideally a refrigeration system should meter just enough refrigerant into the evaporation coil, so that the refrigerant extracts heat throughout the entire length of the coil as it evaporates. Due to the changing dynamics of the system, changes in thermostat setting, changes in load from sun, wind and so forth, the optimal performance is not always easy to achieve.
For example, when the temperature surrounding the evaporator coil is high, refrigerant is rapidly converted from the liquid phase to the gaseous phase and there may not be enough refrigerant in the liquid phase to fill the entire evaporator coil. When this occurs, efficiency suffers, since, in effect, some of the evaporator coil is being wasted. On the other hand, if the temperature surrounding the evaporator coil is low, there may not be enough heat present to cause all of the refrigerant to evaporate from the liquid phase to the gaseous phase. When this occurs, liquid refrigerant may enter the compressor, degrading efficiency and possibly damaging the compressor. Conventional refrigeration systems have employed a number of different control schemes for metering the refrigerant into the evaporator coil. Although purportedly successful to some degree, there still remains a great deal of room for improvement.
With all of these diverse areas for improvement, it would seem desirable to employ microprocessor technology. However, use of microprocessor technology presents its own set of problems, in that microprocessor-based systems tend to employ numerous sensors (with numerous connecting wires) and tend to be very feature-oriented. Seemingly, consumers always want all of the latest features, and thus equipment manufacturers find themselves continually upgrading their product line. When new features are added, although implemented largely in software, frequently the existing hardware will also have to be replaced, at considerable expense.
The present invention takes a different approach to microprocessor-based control systems, in which a highly modular, multiple microprocessor system is provided. The system employs a plurality of processors (e.g. one in the outdoor unit, one in the indoor unit and optionally one in a thermostat unit) and a communications link over which the processors communicate, using a messaging system designed to allow division of labor between processors. The communications link eliminates the need to run a bundle of wires between indoor and outdoor units even though a multiplicity of sensor can be utilized. Moreover, the distributed architecture allows operating functions to be distributed over the plurality of microprocessors so that a portion of the system (such as the indoor unit, or the outdoor unit, or the thermostat) can be upgraded without the need to upgrade the other portions. Also, if a malfunction or component failure occurs in one of the units, the remaining units can in many cases still continue to operate.
The present invention addresses the above efficiency problems in a very effective and inexpensive way. The system employs a variable speed indoor fan which adjusts the airflow across the indoor coil or heat exchanger based on environmental measurements such as outdoor air temperature. The airflow rate across the indoor heat exchanger affects the rate at which heat is extracted (in the cooling mode) or injected (in the heating mode), which in turn affects the superheat within the refrigeration system. The microprocessor-based control system selects the appropriate indoor airflow (fan speed) to place the system in an optimal efficiency range.
To precisely control refrigerant flow, the system provides a microprocessor-based control system and digitally controlled valve. Precise metering of refrigerant is made possible by a decoupled sensing arrangement which is virtually immune from previously troublesome errors caused by changing system dynamics. The system is able to maintain efficient operation at low temperature levels not heretofore readily attained.
The present invention addresses the humidity problem by controlling the speed of the indoor fan to cause a slower airflow when conditions are humid, in order to remove a greater amount of moisture from the air. More specifically, the microprocessor-based control system determines the indoor fan speed to stay within the boundaries of the cooling mode comfort envelope defined by the American Society of Heating and Refrigeration Engineers (ASHRAE), thereby correlating room temperature and humidity. Furthermore, the two-way communication characteristic provided by this distributed architecture approach provides the system designer access to system related information (refer to FIG. 4) in the design of diagnostic functions that enhance system reliability--a feature that may not be readily available using the conventional system design approach.